Outboard motors are often used as a power source for a watercraft such as a boat. In many small-power outboard motors, an exhaust pipe connected to an exhaust port of an internal combustion engine in the outboard motor extends downwardly to such an extent that the outlet opening of the exhaust pipe is submerged beneath the water level in an operable state of the outboard motor. Such configuration can reduce the exhaust noise and/or prevent overheating of the extension case into which the hot exhaust gas is discharged. Although it may be conceived to use an additional pump for lifting up water to cool the exhaust gas, this is often impractical in small-power outboard motors in which power loss resulting from operating the additional pump may be significant and/or there may be little spare room for mounting the pump.
It has been also known in small-power outboard motors to connect the output shaft (or crankshaft) of the internal combustion engine and the propeller via a centrifugal clutch system so that the driving force from the engine is prevented from being transmitted to the propeller when the crankshaft rotation speed is lower than a prescribed value. (See for example Japanese Utility Model Publication (kokoku) No. 60-24714.) This can reduce the necessary starting torque produced from a recoil starter and thereby facilitate the starting of the engine. In such outboard motors, however, a so-called "reverse rotation" of the engine (or crankshaft) tends to take place in the event of failure to successfully start the engine. The reverse rotation lowers the pressure inside the cylinder (or combustion chamber) of the engine, generating a negative exhaust pressure.
Thus, if a reverse rotation of the engine takes place in an outboard motor with the exhaust pipe outlet submerged under water, the engine will work as a suction pump and lift up water. Since the number of reciprocating movements of the piston in a single reverse rotation can be estimated to be fewer than five, if the volume inside the exhaust pipe between the water level and the engine exhaust valve (or exhaust port) is greater than five times of the displacement volume of piston, intrusion of water into the combustion chamber can be prevented. However, it is very difficult to achieve such an exhaust pipe having a sufficient volume in a small outboard motor which has little space to spare.